A number of NixOS options allow a user to specify a file instead of writing one as a nix expression for configuration, e.g. services.usbguard.ruleFile, security.pki.certificateFiles, etc. This can be useful if a user, for example already has a large number of pre-written configurations for whatever program they are using.

In my case, I have a number of Flatpak override config files already present on my device. It would be very useful if I could simply drop them in a folder as part of my OS config. This would also allow users to use tools they are already familiar with, when configuring Flatpak overrides, e.g. flatseal, and the built in configuration tool present on KDE.